This invention relates to an improved method for preparing a coating composition for use the production of a heat-sensitive record material which has an improved heat-sensitivity and is adapted for a high speed recording.
This is known a heat-sensitive record material comprising a base sheet having a color developing layer which includes finely divided particles of one of colorless chromogenic materials such as triphenylmethane compounds, fluoran compounds, auramine compounds and spiropyran compounds and finely divided particles of one of organic acceptors such as phenolic compounds, aromatic carboxylic acids and their polyvalent metal salt and/or one of inorganic acceptors such as activated clay, acid clay, attapulgite, aluminum silicate and talc. In such the heat-sensitive record material like this the above mentioned two kinds of particles are, when at least one of them is melted or sublimated at an elevated temperature, brought into intimate contact with each other to develop a color. Accordingly, a relatively high temperature is required for obtaining clear and distinct color images. This is apparently disadvantageous since clear and distinct color images can never be expected at a high speed recording.
On the other hand, recently, with remarkable advance of the heat sensitive recording machines, it has become possible to speed-up heat sensitive facsimile systems and heat sensitive printers utilizing improved thermal heads. For example, now there are available heat sensitive facsimile systems having the capacity of a recording speed of 20 seconds or shorter for a DIN A4 size paper sheet and heat sensitive printers having the capacity of a recording speed of 120 or more letters/sec. As a result of such speed-up in the field of hardware, there has been a demand for higher sensitivity of heat-sensitive record material to be used. To meet this demand, studies have been conducted to try to find colorless chromogenic materials and acceptors having higher sensitivity, to incorporate a sensitizing agent to the record material; and to obtain particles of colorless chromogenic materials and acceptors having reduced particle sizes through the utilization of improved manners of pulverization.
U.S. Pat. No. 4,236,732 or Japanese Laid-Open Patent Publication No. 48,751 of 1978 proposes to co-melt a colorless chromogenic material or an acceptor with a heat fusible material having such a relatively low melting point as 60.degree. C. to 200.degree. C. The heat-sensitive record material obtained by utilizing this co-melting technique is immediately heat responsive and has a good heat-sensitivity at a low temperature so that it can be used as a record material for high speed recording machines such as facsimiles, electronic computers and telex machines.
The above-mentioned co-melting technique, however, has a disadvantage that the production of a dispersion in which solid particles of a co-melt of a colorless chromogenic material or an acceptor and a heat fusible material are dispersed is not always easy. U.S. Pat. No. 4,236,732 or Japanese Laid-Open Patent Publication No. 48,751 of 1978 discloses two manners for obtaining solid particles of a co-melt of a colorless chromogenic material or an acceptor and a heat fusible material. One is to admix a colorless chromogenic material or an acceptor with a heat fusible material in a co-melted state, cool the mixture and then pulverize the obtained mass into finely divided particles utilizing a ball mill or any other pulverizer. The other is to admix a colorless chromogenic material or an acceptor with a heat fusible material in a molten state and then disperse and emulsify the melted mixture in a dispersion medium. These two manners for obtaining solid particles of a co-melt of a colorless chromogenic material or an acceptor and a heat fusible material are not always practically applicable for any kind of heat fusible material. However, if the target particle size of the particles is set to about 4-5 microns, this can hardly be attained using a ball mill or even if the material is to be processed by a sand mill, at present it must be processed at least 3-4 times. Thus, there are problems that the pulverization treatment requires much labor and time and that despite this time-consuming treatment, the resulting particle size distribution is not always uniform. Further, the equipment itself is complicated and expensive and expenses for treatment operation are substantial, consequently affecting the cost of the heat-sensitive record material. A further problem is that with the equipment in use, it is difficult to prepare the coating composition in a continuous operation.
The primary object of the invention is to provide an improved method for preparing a coating composition including solid particles of a co-melt of at least one colorless chromogenic material or acceptor and at least one heat fusible material in which the above-mentioned disadvantages involved with the conventional techniques can be avoided.
Another object of the invention is to provide an improved method for the production of finely divided solid particles having relatively uniform and relatively reduced particle sizes of a co-melt of a colorless chromogenic material or acceptor and a heat fusible material.
Other objects and advantages of the invention are apparent from the following detailed description.